Lens rolloff assisted auto white balance

Abstract

A method and apparatus for determining an auto white balance (AWB) gain based on a chroma tint pattern resulting from lens rolloff. The method may include receiving an image captured by a camera module, determining a chroma tint pattern of the image resulting from lens rolloff of the camera module, determining an illuminant under which the image was captured based on the determined chroma tint pattern, determining AWB parameters based at least in part on the determined illuminant, and applying the AWB parameters to the image.

What is claimed is: 
     
         1 . A method for image processing, the method comprising:
 receiving an image captured by a camera module;   determining a chroma tint pattern of the image resulting from lens rolloff of the camera module;   determining an illuminant under which the image was captured based on the determined chroma tint pattern;   determining auto white balance (AWB) parameters based at least in part on the determined illuminant; and   applying the AWB parameters to the image.   
     
     
         2 . The method of  claim 1 , wherein determining the illuminant under which the image was captured based on the determined chroma tint pattern comprises:
 comparing the determined chroma tint pattern to one or more predetermined chroma tint patterns, wherein each of the predetermined chroma tint patterns is associated with one of a plurality of predefined illuminants; and   determining the illuminant based on the predetermined chroma tint pattern that most closely matches the determined chroma tint pattern based on the comparison.   
     
     
         3 . The method of  claim 1 , wherein determining the chroma tint pattern comprises:
 applying lens rolloff correction to the image to create a corrected image;   determining a red/green ratio and a blue/green ratio for the corrected image; and   applying a low pass filter to the red/green ratio and the blue/green ratio of the corrected image to determine the chroma tint pattern.   
     
     
         4 . The method of  claim 3 , wherein the low pass filter is a homomorphic low pass filter. 
     
     
         5 . The method of  claim 4 , wherein applying the homomorphic low filter comprises:
 applying a natural logarithm to the red/green ratio and the blue/green ratio to produce a first output;   applying a Fourier transform to the first output to produce a second output;   applying a low pass filter to the second output to produce a third output;   applying an inverse Fourier transform to the third output to produce a fourth output; and   applying an exponential function to the fourth output to produce the chroma tint pattern.   
     
     
         6 . The method of  claim 3 , wherein the lens rolloff correction is a reference light condition rolloff table generated from a reference light condition. 
     
     
         7 . The method of  claim 3 , further comprising:
 applying a black level subtraction to the image before determining the chroma tint pattern.   
     
     
         8 . The method of  claim 3 , wherein determining the red/green ratio and the blue/green ratio for the corrected image comprises:
 determining the red/green ratio and the blue/green ratio for each of a plurality of M×N bins of the corrected image.   
     
     
         9 . The method of  claim 1 , wherein the determined illuminant is a first illuminant, the method further comprising:
 determining a second illuminant using gray-world assumption techniques on the captured image;   determining a first confidence factor for the first illuminant;   determining a second confidence factor for the second illuminant; and   wherein determining the AWB parameters based at least in part on the determined first illuminant comprises:
 determining the AWB parameters based on the first illuminant, the first confidence factor, the second illuminant and the second confidence factor. 
   
     
     
         10 . An apparatus configured for image processing, the apparatus comprising:
 a memory configured to store an image captured by an image sensor of a camera module; and   one or more processors in communication with the memory, the one or processors configured to:
 receive the image captured by the camera module; 
 determine a chroma tint pattern of the image resulting from lens rolloff of the camera module; 
 determine an illuminant under which the image was captured based on the determined chroma tint pattern; 
 determine auto white balance (AWB) parameters based at least in part on the determined illuminant; and 
 apply the AWB parameters to the image. 
   
     
     
         11 . The apparatus of  claim 10 , wherein to determine the illuminant under which the image was captured based on the determined chroma tint pattern, the one or more processors are further configured to:
 compare the determined chroma tint pattern to one or more predetermined chroma tint patterns, wherein each of the predetermined chroma tint patterns is associated with one of a plurality of predefined illuminants; and   determine the illuminant based on the predetermined chroma tint pattern that most closely matches the determined chroma tint pattern based on the comparison.   
     
     
         12 . The apparatus of  claim 10 , wherein to determine the chroma tint pattern, the one or more processors are further configured to:
 apply lens rolloff correction to the image to create a corrected image;   determine a red/green ratio and a blue/green ratio for the corrected image; and   apply a low pass filter to the red/green ratio and the blue/green ratio of the corrected image to determine the chroma tint pattern.   
     
     
         13 . The apparatus of  claim 12 , wherein the low pass filter is a homomorphic low pass filter. 
     
     
         14 . The apparatus of  claim 13 , wherein to apply the homomorphic low filter, the one or more processors are further configured to:
 apply a natural logarithm to the red/green ratio and the blue/green ratio to produce a first output;   apply a Fourier transform to the first output to produce a second output;   apply a low pass filter to the second output to produce a third output;   apply an inverse Fourier transform to the third output to produce a fourth output; and   apply an exponential function to the fourth output to produce the chroma tint pattern.   
     
     
         15 . The apparatus of  claim 12 , wherein the lens rolloff correction is a reference light condition rolloff table generated from a reference light condition. 
     
     
         16 . The apparatus of  claim 12 , wherein the one or more processors are further configured to:
 apply a black level subtraction to the image before determining the chroma tint pattern.   
     
     
         17 . The apparatus of  claim 12 , wherein to determine the red/green ratio and the blue/green ratio for the corrected image, the one or more processors are further configured to:
 determine the red/green ratio and the blue/green ratio for each of a plurality of M×N bins of the corrected image.   
     
     
         18 . The apparatus of  claim 10 , wherein the determined illuminant is a first illuminant, and wherein the one or more processors are further configured to:
 determine a second illuminant based gray-world assumption techniques on the captured image;   determine a first confidence factor for the first illuminant;   determine a second confidence factor for the second illuminant; and   wherein to determine the AWB parameters based at least in part on the determined first illuminant, the one or more processors are further configured to:
 determine the AWB parameters based on the first illuminant, the first confidence factor, the second illuminant and the second confidence factor. 
   
     
     
         19 . The apparatus of  claim 10 , further comprising:
 the camera module configured to capture the image.   
     
     
         20 . The apparatus of  claim 19 , wherein the camera module is part of a mobile telephone. 
     
     
         21 . An apparatus configured for image processing, the apparatus comprising:
 means for receiving an image captured by a camera module;   means for determining a chroma tint pattern of the image resulting from lens rolloff of the camera module;   means for determining an illuminant under which the image was captured based on the determined chroma tint pattern;   means for determining auto white balance (AWB) parameters based at least in part on the determined illuminant; and   means for applying the AWB parameters to the image.   
     
     
         22 . The apparatus of  claim 21 , wherein the means for determining the illuminant under which the image was captured based on the determined chroma tint pattern comprises:
 means for comparing the determined chroma tint pattern to one or more predetermined chroma tint patterns, wherein each of the predetermined chroma tint patterns is associated with one of a plurality of predefined illuminants; and   means for determining the illuminant based on the predetermined chroma tint pattern that most closely matches the determined chroma tint pattern based on the comparison.   
     
     
         23 . The apparatus of  claim 21 , wherein the means for determining the chroma tint pattern comprises:
 means for applying lens rolloff correction to the image to create a corrected image;   means for determining a red/green ratio and a blue/green ratio for the corrected image; and   means for applying a low pass filter to the red/green ratio and the blue/green ratio of the corrected image to determine the chroma tint pattern.   
     
     
         24 . The apparatus of  claim 23 , wherein the low pass filter is a homomorphic low pass filter. 
     
     
         25 . The apparatus of  claim 24 , wherein the means for applying the homomorphic low filter comprises:
 means for applying a natural logarithm to the red/green ratio and the blue/green ratio to produce a first output;   means for applying a Fourier transform to the first output to produce a second output;   means for applying a low pass filter to the second output to produce a third output;   means for applying an inverse Fourier transform to the third output to produce a fourth output; and   means for applying an exponential function to the fourth output to produce the chroma tint pattern.   
     
     
         26 . A non-transitory computer-readable storage medium storing instructions that, when executed, causes one or more processors to:
 receive an image captured by a camera module;   determine a chroma tint pattern of the image resulting from lens rolloff of the camera module;   determine an illuminant under which the image was captured based on the determined chroma tint pattern;   determine auto white balance (AWB) parameters based at least in part on the determined illuminant; and   apply the AWB parameters to the image.   
     
     
         27 . The non-transitory computer-readable storage medium of  claim 26 , wherein to determine the illuminant under which the image was captured based on the determined chroma tint pattern, the instructions further cause the one or more processors to:
 compare the determined chroma tint pattern to one or more predetermined chroma tint patterns, wherein each of the predetermined chroma tint patterns is associated with one of a plurality of predefined illuminants; and   determine the illuminant based on of the predetermined chroma tint patterns that most closely matches the determined chroma tint pattern based on the comparison.   
     
     
         28 . The non-transitory computer-readable storage medium of  claim 26 , wherein to determine the chroma tint pattern, the instructions further cause the one or more processors to:
 apply lens rolloff correction to the image to create a corrected image;   determine a red/green ratio and a blue/green ratio for the corrected image; and   apply a low pass filter to the red/green ratio and the blue/green ratio of the corrected image to determine the chroma tint pattern.   
     
     
         29 . The non-transitory computer-readable storage medium of  claim 28 , wherein the low pass filter is a homomorphic low pass filter. 
     
     
         30 . The non-transitory computer-readable storage medium of  claim 29 , wherein to apply the homomorphic low filter, the instructions further cause the one or more processors to:
 apply a natural logarithm to the red/green ratio and the blue/green ratio to produce a first output;   apply a Fourier transform to the first output to produce a second output;   apply a low pass filter to the second output to produce a third output;   apply an inverse Fourier transform to the third output to produce a fourth output; and   apply an exponential function to the fourth output to produce the chroma tint pattern.